An optical fiber amplifier using an amplifier optical fiber doped with a rare earth element such as Er (erbium) can be expected to achieve high gain but produces an increased amount of ASE (Amplified Spontaneous Emission), which may oscillate by reflection or scattering (Rayleigh scattering) of light. This oscillation is unstable, and the occurrence of the oscillation can cause instantaneous strong light to destroy a pumping source or an optical component. The avoidance of the oscillation requires that the power of pump light be lower than that during the oscillation, thus leading to the limited power of output signal light, and accordingly, desired output power may not be satisfied. Therefore, there is a demand for an optical fiber amplifier in which the oscillation is suppressed.
A conventional optical amplifier designed in consideration for suppression of the oscillation generally uses an isolator. For example, in Patent Document 1, the oscillation by the reflection is prevented over a wide range of temperatures, and also, a filter is used to prevent the oscillation by end reflection at the pumping source.